Summary of Rating of Debt securities (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 15,558,624	$ 16,061,560
BB / Ba
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	130,600
B
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	188,900
CCC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	31,300
CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	500
Below CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	2,900
Not rated securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 90,400